It is the musical term tenuto.

The following is the standard explanation in the music dictionary

The precise meaning of tenuto is contextual: it can mean either hold the note in question its full length (or longer, with slight rubato) , or play the note slightly louder. In other words, the tenuto

mark may alter either the dynamic or the duration of a note. Either

way, the marking indicates that a note should receive emphasis

Depending on the situation, the tenuto mark may be slightly emphasized, or the full duration of the note may be maintained or longer.

The emphasis here does not necessarily mean increasing the volume, but giving special attention (such as making it sound prominent, etc.)

Under normal circumstances (including the pictures given), it can be understood as maintaining integrity. Value, slightly emphasized